1. Field of the Invention
The invention relates to injection molding, and more particularly to dual gate injection molding of articles having portions formed of different thermoplastic materials.
2. Description of the Prior Art
Methods are known for the extrusion or injection molding of laminated structures of mutually different thermoplastic materials, and for sequentially injecting or extruding different plastic materials into a mold or through a die to form products having parts with different characteristics.
Lefevre U.S. Pat. No. 3,528,310 relates to the coextrusion or simultaneous extrusion of a laminate of two or more parallel layers of different thermoplastic materials through concentrically related openings leading to a single extrusion orifice. Sawada et al U.S. Pat. No. 3,754,847 also relates to production of laminates by extrusion using separate passages for different materials leading to a common extrusion orifice.
The injection molding nozzle of Langecker U.S. Pat. No. 3,921,856 is described as simultaneously extruding a filling material and a cover material.
U.S. Pat. No. 3,809,519 to Garner relates to alternating sequential flow of materials through a single injection molding outlet by means of a valve.
Burlis et al U.S. Pat. No. 3,724,985 is concerned with the extrusion of tubing having portions of different materials along its axial length by sequential flow from two molten plastic producers through a single die.
Nye et al U.S. Pat. No. 3,776,674 relates to production of multi-colored articles by moving two separate discharge heads for the different colored materials over an open top mold cavity.
The references discussed all relate to forming articles from more than one material but none shows the use of two gates into a single mold for simultaneous injection of different thermoplastic materials to form a unitary article having portions with different characteristics.
It would be desirable to produce articles having portions with different properties in a single injection molding operation. One article which could be economically produced by such a technique is a unitary plunger for a syringe. Such plungers are usually produced by the mechanical assembly of two or more separately molded parts.
Disposable syringes have generally replaced the much more expensive ground glass syringes formerly used in medical practice, but the manufacture of such disposable syringes from plastic materials has involved certain difficulties.
The barrel of a syringe is of a length considerably greater than its radius, so in order to assure release of the barrel from an injection mold the barrel is formed with a slight taper from its open end to its closed end. A rigid cylindrical plunger sized to fit snugly within the open end of such a tapered barrel will become tightly wedged in place when advanced toward the closed end of the barrel and will lock in position before discharging the barrel contents through a cannula at the closed barrel end. Differential shrinkage of injection molded thermoplastic barrels results in inward bowing of the middle portion of the barrel between the barrel ends and thus further departure from uniform interior cross sectional area.
To overcome these irregularities of dimension, plungers for injection molded syringe barrels have been made with compressible gaskets or discs at their head ends to accommodate variations in barrel diameter and to avoid leakage around the plunger. Then discs or gaskets are typically made of rubber or other elastomeric material and then fitted on the barrel by stretching and snapping the elastomeric piece into place. The manufacture and assembly of two separately formed parts obviously tends to be less economical than would a technique of manufacture which required no assembly.